This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Cytokinesis beginning in early anaphase represents the last stages of mitosis where two daughter cells become separated via an event termed abscission. As the daughter cells progress through cytokinesis they form a cleavage furrow that ingresses by the action of an actomyosin contractile ring. This forms a narrow cytoplasmic bridge that needs to be resolved to produce two separate daughter cells. Additionally, it has been shown that recycling endosomes are required for successful cytokinesis and abscission. Unfortunately, the mechanism and function of endosomes at the cleavage furrow remain unclear. Two hypotheses have been proposed that try to explain the role of endosomes during cytokinesis. The first hypothesis, a luminal filling model, suggests synchronized endosome fusion at the furrow mediates the scission of daughter cells. The second hypothesis, a delivery model, proposes a function for endosomes in the regulation of actin cytoskeleton dynamics and/or lipid composition at the furrow through the delivery of endosomal vesicles. Previous work from our laboratory and others have established a role for recycling endosomes in mediating late stage cytokinesis. Rab11 and FIP3 have been shown to be involved in targeting of recycling endosomes to the cleavage furrow during cytokinesis . To determine the spatiotemporal dynamics of Rab11/FIP3 associated endosomes within the cleavage furrow, we will use electron tomography to three dimensionally reconstruct the cleavage furrow. Comparison of early and late telophase cleavage furrow tomograms will yield a sense of what organelles populate the furrow during telophase. Additionally the use of immunoEM will provide for the identification of organelles populating the cleavage furrow.